Insulating tapes, which are known, for instance, from German Patents Nos. 18 01 053, 21 42 571 and 30 03 477 for the impregnation with epoxy resin/acid anhydride hardener mixtures, make it possible to carry out the impregnation economically since a slow-reacting impregnating resin can be used which is practically subject to no change of the viscosity, also at impregnating temperatures which assure a thorough impregnation of the insulating sleeve. This is necessary because only a small percentage of the reaction resin mixture used as the impregnating resin penetrates into the insulating sleeve while the rest must be reused for a new impregnation. For this reason, the insulating tape contains an accelerator which stimulates the hardening reaction of the impregnating resin so that the share of the impregnating resin which has penetrated into the insulating sleeve hardens in an economically justifiable time.
According to German Patent No. 18 01 053, the bonding agent/accelerator mixture is chosen so that in the insulating tape practically no hardening of this mixture takes place at room temperature. In this way the insulating tapes can be stored for a long time before being impregnated. In addition, the bonding agent/accelerator mixture forms a self-hardening system which self-hardens only at hardening temperatures of the impregnating resin which are substantially higher than room temperature. In this way it is avoided, that bonding agent which has penetrated between large-area layers of the breakdown-proof inorganic material and which could not be resorbed by the impregnating resin completely, remains in the insulation unhardened.
In the insulating tapes according to German Patent No. 21 42 571, cycloaliphatic epoxy resins are used as the bonding agents, in which the epoxy groups were generated by oxygen addition at ring-located double bonds and which, as is generally known, exhibit no or only strongly delayed reactions with amino hardeners or accelerators. If the epoxy/acid anhydride impregnating resin is added, these accelerators, however, develop their full catalytic activity. Glycidyl ethyl, glycidyl ester or N-glycidyl epoxy resins do not exhibit these different reactivities. However, cycloaliphatic epoxy resins can be prepared only with difficulty. In addition, according to more recent investigations, suitable, cycloaliphatic epoxy resins have certain adverse physiological effects.
These disadvantages are taken into account in the insulating tape according to German Patent No. 30 03 477 by the provision that quaternary onium salts are added as accelerators to the glycidyl ether resins used as bonding agents. Insulating tape constructed in accordance with this state of the art can be stored at room temperature for more than three months. However, the usability of the tape can be limited through moisture absorption of the tape due to the hygroscopic nature of the onium salts.
The mica tapes known from the German Patent No. 18 01 053 mentioned above are insulating tapes which have as bonding agents, polymerizable adducts of acid components, which contain at least one radically polymerizable double bond, and epoxy compounds. It is possible to anchor the bonding agent in the heat-hardening impregnating resin system with an epoxy resin base via the hydroxyl groups generated during the addition. By adding suitable organic peroxides to the bonding agent, the double bonds capable of being polymerized can be polymerized radically, whereby self-hardening of the tape adhesive is achieved. As an accelerator for the impregnating resin, this tape adhesive contains zinc salts of unsaturated polymerizable acid compounds.
However, the hardened pure, adhesive resin and the molded material of heat-hardening impregnating resin and mica tape adhesive generated in the hardening, known from the mentioned state of the art, have unsatisfactory thermal-mechanical properties. Thus, a typical molded material of pure adhesive resin such as is described in German Patent No. 18 01 053, has a Martens temperature of about 95.degree. C. The value for a molded material of a suitable epoxy/acid an hydride impregnating resin mixture and the tape adhesive is only slightly higher. The known adhesive resins or the mica tapes made therefrom are therefore suited only for making insulating sleeves of insulation class B.
In rotating machines, the stiffness of the end face insulation contributes to the overall stiffness of the coil head and thus codetermines deformations of the coil head, for instance, in the case of switching and of short circuits. However, the smaller that these deformations are (i.e., the stiffer the insulating sleeve is, the smaller is the danger of damage due to the formation of cracks in this region.
The present state of the art is exemplified, for instance, by high voltage motors which can be stressed thermally up to 155.degree. C. (insulation class F). However, reaction resin molded materials with Martens temperatures &lt;100.degree. C. are not well suited for this purpose, since expensive additional measures for reinforcement are necessary.
It is an object of the invention to provide an insulating tape with a tape adhesive with an epoxy-free base which leads, with heat hardening epoxy impregnating resins, to an insulating sleeve with improved thermal-mechanical properties, particularly greater stiffness.